1. Field of the Invention
The present invention relates to a processing method, and apparatus using plasma generated by electron cyclotron resonance (hereafter abbreviated to ECR), and in particular to a plasma processing method, and apparatus, suitable for film forming, strong anisotropy etching, surface refining or plasma doping comprising processing while impinging ions from plasma on a substrate.
2. Description of the Related Art
In conventional plasma processing apparatuses, especially those proposed to enhance the ion processing efficiency, as described in JP-A-56-13480 and JP-A-63-197327, means is provided for generating plasma and a high-frequency electric field having a frequency that ions can follow is applied to a substrate to be processed. Ions are impinged on the substrate by the high-frequency electric field, the substrate being thus processed.
The above described prior art attempts to improve the processing efficiency and uniformity of a plasma processing apparatus by applying a high-frequency electric field to the substrate to impinge ions on the substrate with a frequency which can be followed by the ions. However, control of an induced DC potential generated in the substrate on the basis of discharge caused between the substrate and a processing chamber by the high-frequency electric field applied to the substrate or the difference in mobility between the electrons and ions in plasma is not considered. Even if a frequency which could be followed by ions was determined as the high frequency applied to the substrate, a surplus DC potential would be induced in the substrate. This results in a problem that discharge caused between the substrate and the plasma by this surplus induced potential might severely damage the substrate. In a fabrication process for semiconductor devices, there was posed a problem that element characteristics were degraded by storage of a charge due to induced potential. Further, if power applied to the substrate was increased to improve the plasma processing efficiency, the potential induced in the substrate was also raised accordingly. This resulted in a problem that improvement of processing efficiency was prohibited.
Further, when discharge was caused in the plasma by the high-frequency electric field applied to the substrate, the gap between the substrate and a substrate holder to which the high-frequency electric field was applied formed a large impedance component in this frequency region. This resulted in a problem that uniformity of plasma processing in a substrate or between substrates was poor.
These problems will now be described in more detail.
When a high frequency is applied to a substrate, electrons and ions contained in a plasma are moved in a direction nearly perpendicular to the substrate by the high-frequency electric field. As well known, however, there occurs a difference between the amount of electrons impinged on the substrate and the amount of ions impinged on the substrate because of a difference in mobility based upon mass and a difference in frequency of collision based upon the diameters of particles.
The above described difference becomes small when a frequency which can be followed by the ions (such as a frequency of 1 MHz or less in case of N.sup.+ and O.sup.+) is applied. By the difference in amount of impingement, however, a DC potential which is not higher than 20 V in absolute value with respect to the plasma potential is induced in the substrate. On the other hand, electrons and ions are forced to move by the high-frequency electric field and collide with other particles. When power applied to the substrate is large, the acceleration of electrons and ions is also large and collided particles are ionized, resulting in momentary discharge. If at this time electrons or ions flow out of the plasma and the plasma is in contact with the internal wall of the processing chamber coupled to the ground potential, for example, the electrons or ions flow into the ground through the internal wall and hence electron avalanche is caused, high-frequency discharge being thus maintained. If electron avalanche occurs, the ratio of the above described amounts of impingement changes little. Since the absolute value of the difference in amount of impingement increases exponentially, however, the DC potential induced in the substrate amounts to -10.sup.2 to -10.sup.3 V. When discharge occurs, power applied to the substrate is consumed in order to maintain the discharge, i.e., as a current flowing to the substrate and the internal wall of the processing chamber via the plasma. As a result, the DC potential induced in the substrate becomes high. However, the high-frequency potential applied to the substrate decreases in proportion to the power consumed by the discharge. If new discharge is caused by the high frequency applied to the substrate, a highly negative DC potential causing damage of the substrate is induced. If the DC potential induced in the substrate becomes high in absolute value, a local discharge corresponding to a so-called falling of a thunderbolt may occur between the plasma and the substrate and severely damage the substrate, or charge up of the substrate based upon the induced DC potential may cause damage or deterioration of elements formed on the substrate, resulting in problems.